Coating metals by the aid of acid sulphites



Patented Mar. l6,'1943 V ooarme mum s ax 'rna Am OF ACID surrm'ras v Iao P. Curtin, Oranbnry, N. .L, assignor', byniesne assignments, to Parker Bust-Proof Company,

Detroit, Mich, a corporationof Michigan No Drawing. Application April 16, 1937,-

Serial No. 131.376

. 9 Claims. (01. 148-6) This invention or discovery relates to coating metals by aid of acid sulphites; and it comprises a process of forming adherent thin continuous coatings upon surfaces of iron or steel. zinc and similar metals wherein the metal surface is treated with an aqueous solution containing a metal acid sul-phite advantageously acid zinc sulphite, Zn(HSO3)z reacting with the surface metal to form thereon a coating of insoluble metal com-' pounds containing oxides and sulphides; and it further comprises a material for use in a solution for providing bonding coats on metal, including as the major constituent a bisulphite and sometimes in addition a salt of an acid the primary hydrogen ionization constant of which is intermediate the primary and secondary ionization constants of sulphurous acid.

Furnishing metal surfaces with bonding or foundation coats for-applications of paint, lacquer and enamel has become a matter of considerable commercial importance. In the automobile industry, for example, the finish of the car body depends upon the foundation coat acting as a bond between sheet metal and the paint, lacquer or enamel finish. The bonding coat may or may not be of a protective character per se, but in all cases is adapted to form a secure bond oxide or sulphite; and

(b) A metal in solution which is capable of entering the coating in the form of insoluble (c) A small concentration of bisulphite or other sulphur compound capable of reacting with the metal to be coated to form oxide and sulphur compounds thereof.

The acid concentration may vary in the approximate limits of pH2 to pH5, more or less, which means that a small concentration of sulphurous acid is present. In the presence of too much acid, no coating, or a non-adherent coating, will form and if the solution becomes much less acid than pH5, the coating action is retarded and there may be precipitation of sludge in the coating bath. For the metal entering the coating from the bisulphite solution, zinc, as I have found, gives the best coating, and in this respect it is followed in order by iron, manganese, cadmium, magnesium, aluminum and trivalent chromium. In general for best results the metal to the metal on the one side and to the paint on the other. These bonding coats have been produced in various ways. In one method metals, such as iron and zinc, are treated with solutions of various metal acid phosphates forming coatings of insoluble secondary and tertiary metal phosphates. In another method the metal is treated with a solution of ferric oxalate reacting with the metal to form an adherent coating containing ferrous oxalate. These metal coatings of the prior art all have specific advantages and disadvantages. It is an object of the present invention to provide metal coatings of a different type having certain practical advantages.

I have found that an excellent bonding coat should be lower than magnesium in the electromotive series, although magnesium itself dissolved in the bath gives a fair coating. To supply zinc and also the other metals named to the bath, it is usually most convenient to use the sulphate of the metal. Nitrate is also satisfactory. Coatings may be formed with acetate or chloride or other salts of acids which are notincompatible with the other components of the bath. I find the sulphates most advantageous having corrosion inhibiting properties is quickly formed upon metal surfaces by action of a solution containing a bisulphite (usually sodium bisulphite) or an acid thiosulphate and a metal salt. The metabisulphites and pyrosulphites act similarly to bisulphite. Heating the solution accelerates the coating action. One or two minutes immersion of the metal article in the coating bath is usually sufficient. The metal acid sulphite solution can also be applied advantageously as a spray. For the coating of iron or steel, the coating bath should contain (a) A small concentration of acid;

because of their cheapness, and because the coating baths formed by the sulphates of the various metals desired in the coating give excellent coatings.

Salts of more than one metal may be used, as for example, mixtures of sulphates of zinc, ferrous iron and divalent manganese. After a bath has been used for coating ferrous metals, it always contains a small amount of ferrous salt, if iron is being coated the iron of which is derived from the material being coated.

For supplying combined sulphur to the coating, the bath most advantageously contains an alkali metal bisulphite, sodium, bisulphite being most convenient. Sodium thiosulphate may be used in a weakly acidified bath, but is less desirable than bisulphite. A solution of the metal'salt and sodium bisulphite gives an action upon the metal to becoated equivalent to that of a metal bisulphite or acid sulphite. For example, a soluis reducedand the metal is oxidized. Little or no hydrogen is evolved. The coating formed upon the metal contains oxlcles of the coated metal and of the metal dissolved in the bath. It also contains sulphur in combination as a sulphide.

A bath which is simple, efficient, quick in action and inexpensive to make, and which gives continuous thin coatings adhering with great tenacity to the underlying metalcoatings which are not impaired by bending or stamping the coated metal-is made up of the following composition:

Parts by weight Sodium bisulphite, NaHSO: 30 Zinc sulphate, ZnSO4.7I-IzO 90 Water 880 Under some circumstances, it is desirable to increase somewhat the acidity of the solution.

over that provided by bisulphite. This is best done by adding about one-tenth per cent concen-- trated sulphuric acid on the weight of the bath or an equivalent amount of diluted acid. The addition of approximately one quarter per cent of sodium bisulphate is a convenient way of adding the additional acid. In any case, a clear water white solution is formed. The solutions carrying added acid are advantageous in treating certain types of cold rolled sheet iron, where the surfaces are less readily attacked by bisulphite alone.

Iron, zinc, or a similar metal immersed in or sprayed with the hot solution receives a continuous thin coating in seconds. In one or two minutes the coating is sufficient. The metal surface should be clean. The metal is usually degreased before treatment. Acid pickling is usually unnecessary except when the metal to be coated carries considerable rust or scale. The bath can be operated for quick work at temperatures as high as 80-95 C. Satisfactory coatings are obtained in one to two minutes at 30 to 50 C., the latter temperature, 50 0., being recommended for commercial installations. At lower temperatures the coating action is slower. Under normal conditions of acidity no sludge precipitates in continuous use of the bath.

In cases where the bath becomes too low in free acid, this is best corrected by additions of small amounts of sulphuric acid or sodium bisulphate, as above described. If desired, the bath may be made and replenished by dissolving zinc oxide in sulphurous acid and adding the resulting zinc acid sulphite directly to the bath.

The concentrations of the bath components are not at all critical. For example, zinc sulphate may vary from 4 per cent to per cent and sodium bisulphite from per cent to 4 per cent.

Good coatings are obtained with a bath containing only zinc sulphate and sulphurous acid, 9 per cent ZnSO4.7H2O and 0.5 per cent S02 being a typical formula.

A desirable bath for commercial use is made up as follows (percentages by weight on the bath) Per cent Zinc sulphate, ZIISO4.7H2O 9.0 Sodium hydrogen sulphate, NaHSOr 0.75 Sodium hydrogen sulfite, NaI-ISO3-- L 1.75 Sodium acetate, NaCzHsoz 0.75

Because of the presence of the buffering salt, so-

.dium acetate, this bath contains very little free sulphurous acid; nearly all the tetravalent sulphur, which is the active coating agent, being present as bisulphite. The bath owes its acidity principally to free acetic acid which, in small concentrations, does not impair the quality of the coating while apparently increasing somewhat the rate of formation of the coating. Such a bath contains no normal sulphite and therefore does not throw down zinc normal sulphite. Other buffering salts may replace acetate, for example, formates and lactates. These may be salts of potassium, ammonium, zinc, magnesium or any other positive ions which are compatible with the bath and which do not impair the quality or formation of the coating. In general, the buffering salt should be one of an acid the ionization constant of which is intermediate between the primary and secondary ionization constants of sulphurous acid, H2803.

The sulphurous acid or bisulphite in the coating bath acts upon the metal and is reduced, and the zinc contained in the bath enters the coating. The chemical reactions taking place are not precisely known and the proximate composition of the coating is not exactly known.

The principal reactions in the coating of iron are believed to be typified as follows:

These reactions are by way of illustration only and, as will be evident, they may vary through a wide latitude with respect to the products formed.

Where thebath contains a great deal of acid, reaction (B) is quite noticeable: with less acid and especially with the bufiered baths, reactions of the type of (A) appear to be the principal ones. Any hyposulphite formed may be easily reconverted into coating material by oxidation, especially with atmospheric oxygen, for example, by blowing air through the bath. The reaction may be considered to take place as follows:

In cases where the bath is used as a spray, it is usually found that there is suflicient contact with air to oxidize hyposulphite as fast as formed.

When an iron metal surface is coated by action of a bath containing the equivalent of zinc bisulphite, the coating contains oxidized zinc, ferrous oxide, and sulphide. It appears that zinc oxide and zinc sulphide are present as such in the coating. When the coating is quickly stripped from the metal in an acid solution the odor of hydrogen sulphide is quite pronounced and silver nitrate solution gives a precipitate of black silver sulphide in the stripping solution. The same tests on the same material before coating are negative. The bath solution is found to be free of sulphide. It appears that the sulphide content of the coating is substantial and that the metal constituents of the coating are present largely as oxides. It is believed that the coating contains complex metallic oxysulphides.

When zinc surfaces are coated with the bath containing zinc and sulphurous acid or zinc bisulphite, the reactions taking place are apparently similar to those associated with the coating of iron by the same bath. The coating appears to contain zinc oxide and zinc sulphide as proximate constituents. It is possible however that other sulphur compounds are formed to some extent. The coating solution after acting upon the metal contains true hyposulphite or hyposulphurous acid, H2S204.

The coating reaction on zinc may be repre- It has been found that a 2 per cent solutionof sodium bisulphite attacks iron and a coating is formed. The coating first formed, however, may

be discontinuous and in part non-adherent. After the bath is operated for some time,it contains dissolved iron and the coating improves. Zinc metal, when treated'with sodium bisulphite or sulphur dioxide solution takes an excellent thin, continuous, adherent coating forming a good bonding undercoat for paint or lacquer.

It has sometimes been found helpful in coating the metals to have in the coating solution a trace of a metal capable of plating out on the metal to be coated. This is hardly necessary with bisulphite baths, but sometimes the coating ac-- tion is helped by adding to the bath 1 to 5 parts of copper sulphate per million parts -solution by weight. Because of the sulphurous acid in the bath, itwill not tolerate more than a small content of a cupric salt.

When desired, a small proportion of phosphate can be provided in the coating bath which adds a metal phosphate to the coating which may then contain sulphides, metal oxides and phosphates or basic phosphates. An addition of phosphate to give a bath concentration of 0.5 to 1.0 per cent P205 is efiective in putting a metal phosphate into the coating. The P205 may be added as phosphoric acid or as a soluble dihydrogen phosphate. In the latter case it may be necessary slightly to increase the free acid in the bath to avoid precipitation of insoluble phosphates. The coating with addition of phosphates therein has a somewhat rougher texture than the coating formed by sulphites, and may be desirable with certain types of paint or lacquer.

The coating produced on zinc surfaces by the processes herein described hold paint and lacquer so perfectly that there is usually no need of any further treatment. Where the zinc bisulphite treatment is applied to ferrous surfaces, I have found that the adherence of the lacquer film and the resistance to rusting by the finished article is substantially improved if, before lacquering, the iron or steel surface carrying the coating is dipped for a short time, ten seconds to one minute, in a solution containing the chromate radical. After removal of the object from the'solution, it is most advantageous to avoid rinsing and to dry the adhering liquor of the chromate solution on the surface by exposing it for a short time to an elevated temperature or to a strong current of air. For this treatment, a simple chromic acid solution of 0.10 to 0.50 per cent concentration is satisfactory; usually a 0.25 per cent chromic acid solution. The coated object may be dipped in this solution, either hot or cold, for periods ranging from a few seconds to several minutes. It is then removed and the adhering chromic acid solution is dried on by means of a current of hot air.

I have found that the addition of certain other chemicals to the chromic acid bath improves somewhat the value of this chromating treatment. For example, there may be added to an 0.25 per cent chromic acid solution any of the following: 0.05 per cent sulphuric acid, '0.10' per cent phosphorlc acid, 0.10 per cent oxalic acid, 0.20 per cent zinc chromate, 0.20 per cent ammonium dichromate, 0.50 per cent zinc sulphate.

I have also found that two or three tenths per cent solutions of certain other materials, simisalts especially basic chromic nitrate and chromic chromate. The reasons why these various rinses improve the quality of the coating are not definitely known, but repeated experiments leave no doubt that such benefit is actually obtained.

The coatings containing sulphide withstand high temperatures better than phosphate coatings which usually commence to decompose at temperatures of 200 C. and less. Zinc sulphide, as is known, is oneof the most stable sulphides with respect to oxidation and at high temperatures. The metal coatings containing zinc as oxide and sulphide have been exposed to'a temperature of 400 C. for 30 minutes with'no apparent ill effects. These coatings which are stable at high temperatures are valuable in the vitreous enameling industry where the matter of preparing sheet iron to make a good bond with fused vitreous enamel is always a serious one. For purposes of enameling iron, the metal may be processed by the present invention, receiving a sulphide, or sulphide plus oxide coating. Vitreous enamel is then fused on and 'a perfect bond is formed between the metal and the enamel.

An acidulated solution containing a zinc salt and anoxidizing agent forms upon an iron surface an excellent bonding coat containing oxides of zinc and of iron. In the present invention the bi'sulphite or sulphurous acid in coating the metal is reduced and thus has an oxidizing action. In addition, the sulphite has the efiect of forming as part of the coating a metallic sulphide. An oxidizing agent such as a nitrate in small amount may be added to the acid sulphite sulphite, until a visible protective coating is ob-,

tained on the metallic surface and thereafter treating the coated surface with a solution of chromic acid.

2. A method of producing adherent protective and bonding coatings on surfaces of iron, steel and'zinc which comprises contacting the metal surface with an acidulous aqueous solution having as its principal reactive ingredients the constituents of an acid sulphite of at least one metal not above magnesium in the electromotive series and containing a minor amount of buffering salt of an acid having an ionization constant intermediate between the primary and secondary ionization constants of sulphurous acid and thereby forming-on said surface a coating consisting principally of oxides and sulphides of the metal of said surface and the metal in said solution.

3. The method of claim 2, wherein the said solution contains from 4 to 20 per cent of a zinc salt, from 0.5 to 4 per cent of sodium bisulphite, and a minor amount of a buflering salt selected from the class consisting of acetates, formates, and lactates. 4. A solid material comprising ingredients soluble in water and which, when dissolved in water, yield a small concentration of acid, a metal below magnesium in the electromotive series and which is capable of entering the coating in the form of an insoluble compound of the class consisting of oxides and sulphides, and a small concentration of a sulphur compound capable of reacting with the metal to be coated to form oxide and sulphide compounds.

5. A solid material for use in a metal coating solution, said material consisting essentially of soluble sulphite and soluble sulphate.

6. A solid material for use in a metal coating solution, said material consisting essentially of portion of said powder being-an acid salt.

7. A solid material for use in a metal coating solution, said material consisting essentially of soluble sulphite and soluble sulphate, the base elements in said saltsbomprising a metal below magnesium in the electromotive series.

8. A solid material for use in a metal coating solution, said material consisting essentiallycf soluble sulphite and soluble sulphate, the base elements in said salts comprising zinc.

9. A solid material for use in a metal coating solution, said material consisting essentially of 30 parts sodium bisulphite, '90 parts zinc sulphate and 2 /2 parts sodium bisulphate.

LEO P. CURTIN. 

